The present invention generally relates to electrical systems. More particularly, the invention involves enabling an electrical device rated at a particular voltage to function in a system supplying a higher voltage. In one implementation, the invention may involve using integrated starter-alternator electronics as a pulse-width modulation drive for a conventional starter of an internal combustion engine.
Several integrated starter-alternator (ISA) systems have been proposed for providing starting and charging functions to an internal combustion engine. Such ISA systems are often driven by a crankshaft, belt, or chain. One configuration of a typical belt driven ISA system is illustrated in system 10 of FIG. 1. As illustrated, system 10 may include an inverter (102), which is connected to a direct current (DC) voltage source (e.g., battery 105), that drives an alternating current (AC) electrical device, such as starter-alternator 115, during engine cranking. After the engine (101) starts, the same electronics provide rectification for charging the DC voltage source.
Often, ISA systems are vital to engine stop-start systems. A stop-start system may be used to shut off an engine during prolonged idle periods and restart the engine in response to changes in throttle or clutch position. Consequently, start-stop systems can be used to reduce emissions and fuel consumption. However, a typical start-stop system for an internal combustion engine in a vehicle may start an engine 500,000 times over a 150,000 mile lifetime. This high cycle requirement is prohibitive for cranking with a conventional starter. In contrast, ISA systems are well suited to the task since they are brushless and designed for continuous operation.
In addition to durability, stop-start systems typically require ISAs to have high crank speeds to keep start-up emissions low. High crank speeds are also needed to minimize starting times and to avoid noticeable lag times in, for example, traffic flow. This high crank speed requirement translates into a high starter power output requirement.
In automotive applications, conventional starters output between 1.4 to 1.7 kW. However, stop-start systems require ISAs that output 4 to 8 kW. Thus, despite the higher efficiency of ISAs (typically 75-85% compared to 50% for conventional starters), higher battery power is required for start-stop systems. A start-stop ISA system may require a battery with three times the available power of a conventional starter. A typical ISA battery is 36 volts (V), compared to 12V for a conventional starter. The higher voltage system allows more power to be delivered at the same current using the same cable size.
There are, however, several difficulties involved in using ISAs (especially belt driven systems) for cold engine cranking. It is difficult, due to size, to package ISAs, which can provide adequate cold crank torque, as an accessory on existing engines. Also, the mass moment of inertia of a large rotor in such an ISA system produces high belt loads and increased fuel consumption during acceleration. Additionally, in order to transmit cold crank torque, a higher than normal belt tension is required. As a result, a wider belt and larger bearings are required in the engine and belt loop components.
A conventional starter can be added to a start-stop ISA system to provide the cold cranking ability. The number of cold starts over the life of a vehicle is well within the durability limit of a conventional starter. As FIG. 2 illustrates, a conventional starter 210 can be coupled to, or included in, system 10. The conventional starter is used for cold cranking and is powered by a standard 12V cranking battery with relatively high cold cranking amps and low reserve capacity (e.g., battery 209). For warm cranking, when cranking torque is low, the ISA is used with a high power 36V battery (e.g., battery 105).
As depicted in FIG. 2, start-stop ISA systems employing conventional starters often include a 12V battery (209) to drive the starter. However, a typical 12V battery has low reserve capacity, which is prohibitive for powering certain loads, such as lamps and radios. Further, an extra 12V battery adds weight to a vehicle and consumes valuable space. There are system architectures in which the 12V battery is eliminated and all loads are powered by a 36V battery. However, powering a conventional starter directly from a 36V requires matching the battery power to the power rating of the starter.
Moreover, it is not possible to make an equivalent size conventional starter capable of handling a 36V battery sized for a stop-start ISA system. Even if the conventional 12V starter were rewound for 36V, the resulting high current draw would damage the starter by overheating, demagnetization, and/or contact welding. For these and other reasons, it is beneficial to utilize ISA electronics to drive a conventional starter.
The present invention is directed to methods and systems that substantially obviate one or more of the above problems and other problems by enabling an electrical device rated at a particular voltage to operate in an electrical system providing a higher voltage. This may be accomplished without an additional low power battery and without matching the existing battery power to the power rating of the device. Although the present invention, in its broadest sense, is not restricted to start-stop ISA systems, such systems are used here to convey aspects of the invention.
One aspect of the instant invention involves generating a reduced average voltage from an electrical system. The instant invention may, for example, enable a conventional 12V starter to operate in a start-stop ISA system having a 36V voltage source.
Systems consistent with principles of the instant invention may comprise a combination of elements including an electrical device rated at a particular voltage, an AC load, and a voltage source supplying a voltage higher than that at which the electrical device Is rated. In one implementation, the electrical device could be a starter mechanism coupled to, or included in, a start-stop ISA system and used for cold cranking an internal combustion engine. The starter mechanism may include a DC motor rated at a particular voltage, for example, 12 volts. Consistent with one implementation, the AC load could be a poly-phase starter-alternator mechanism configured for warm cranking the engine, i.e., cranking the engine after periods of extended Idle in response to changes in clutch or throttle position. The starter-alternator may also convert rotational energy produced by the engine into an AC current in order to charge the voltage source and/or provide power to other devices. The starter-alternator mechanism may require, and therefore the voltage source may provide, a voltage higher than that at which the starter is rated (e.g., 36V).
Consistent with principles of the present invention, a reduced average voltage may be provided to the electrical device by way of an electrical circuit. For instance, the voltage source may supply 36 volts and the electrical circuit may provide 12 volts to the device. This reduced average voltage may be produced via one or more switching devices. In one configuration, the switching devices may be included in an inverter/converter circuit coupled to an ISA system. In addition, the electrical circuit may provide the reduced average voltage in response to a switch (e.g., an electromechanical switch) triggered by a key-driven or push-button starter switch.
In addition to providing the reduced average voltage, the electrical circuit may be configured to drive an AC load of a chosen frequency and phase. That is, the circuit may be configured to provide an adjustable-frequency alternating current to the AC load from the DC voltage source. In one configuration, the circuit may, in response to the switch opening, cease to provide the reduced average voltage to the electrical device and transfer energy from the DC voltage source to the AC load. Consistent with one implementation, where the AC load is a starter-alternator device, the electrical circuit may also enable the DC voltage source to be charged via AC current obtained from the engine rotation.
Additional aspects related to the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or maybe learned by practice of the invention. Aspects of the invention may be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing and the following descriptions are exemplary and explanatory only and are not intended to limit the claimed invention in any manner whatsoever.